Solder-precoated conductor circuit substrate and method of producing the same

ABSTRACT

It is to establish a solder precoated conductor circuit substrate capable of conducting the fine pitch mounting and being excellent in the productivity and a technique of producing the same. In the solder precoated conductor circuit substrate, the solder layer formed on the conductor for the connection of electronic component is constituted with Sn thin film layer formed by Cu-Sn substitution reaction based on, for example, the Cu complex formation of thiourea and a Pb-coated Sn layer formed by covering at least a part of Sn crystal grains formed through Sn unhomogeneous reaction based on selective precipitation on Sn with Pb through Sn-Pb substitution reaction based on ionization tendency, in which the solder layer is desirably heated and melted and thereafter cooled to form an alloy layer.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to a solder-precoated conductor circuit substrateand a method of producing the same, and more particularly to asolder-precoated conductor circuit substrate capable of mountingcomponents in a high packaging density and having an excellentproductivity and a method of producing the same.

2. Background Art

Recently, conductor circuit substrates for mounting electroniccomponents such as IC, LSI and the like are demanded to have a finepattern with the miniaturization and high densification of theseelectronic components. For this purpose, more precision is required evenin the connection of surface mounting components to the conductorcircuit substrates.

As a technique of connecting the surface mounting component to theconductor circuit substrate, there is widely used a soldering method,particularly a reflow soldering method with a high productivity. Thismethod is a technique in which a solder paste is previously suppliedonto a copper foil (pad) located on a surface of a printed circuitsubstrate and a surface mounting component is positioned and mountedthereon and thereafter the resulting assembly is placed in a heatingfurnace (fellow furnace) to melt the solder for the connection.

(a) In this technique, a molten solder plating method, a solder pasteprinting method, an electrolytic solder plating method, an electrolesssolder plating method and the like are adopted as a method for supplyinga solder for surface mounting onto the pad of the conductor circuitsubstrate.

However, these solder supplying methods have the following problems forconducting the surface mounting of fine pitch.

(1) In the molten solder plating method, it is difficult to control theplating thickness and a uniform plated film is not obtained.

(2) In the solder paste printing method, the limit of pitch widthcapable of supplying the solder is 0.3 mm and the narrowing of the pitchcan not be conducted.

(3) In the electrolytic solder plating method, a lead for voltageapplication is required and hence the steps become complicated and thedegradation of pattern wiring density results.

(4) In the electroless solder plating method, the formation of platedfilm is a substitute on reaction with copper and it is difficult toincrease the thickness of the film.

Furthermore, there is a method in which Sn film and Pb film formed onthe pad of the conductor circuit substrate through plating is alloyed tofeed a solder for surface mounting as the other method of supplying thesolder (JP-A-2-101190 and JP-A-4-21795). According to this method, thesolder having a desired alloying ratio can easily and surely be providedon the pad of the conductor circuit substrate.

However, the latter solder-supplying method has the following problemsin order to conduct the surface mounting of fine pitch:

(1) When Sn film and Pb film are formed by electrolytic plating, a leadfor voltage application is required and hence the steps becomecomplicated and the degradation of pattern wiring density results.

(2) When Sn film is formed by electroless plating, it is difficult toincrease the film thickness.

(3) Since Sn film and Pb film are formed in the form of individuallayers, it is difficult to conduct complete alloying in the reflowsoldering.

(b) On the contrary, a super-solder technique and a self-solder QFPtechnique are recently proposed as a soldering technique in the surfacemounting. That is, the super-solder technique is a solder producingtechnique in which an alloy obtained by heating reaction between Pb andSn of organic acids is selectively precipitated onto the pad of theconductor circuit substrate. On the other hand, the self-solder QFPtechnique is a technique in which outer lead (pin) portions of thesurface mounting component is previously subjected to a high-speedelectrolytic solder plating and mounted onto the conductor circuitsubstrate. Certainly, it is possible to conduct the surface mounting offine pitch onto the conductor circuit substrate according to thesetechniques.

However, all of these conventional techniques have a problem that theproductivity is poor. That is, the super-solder technique is high in theproduction cost, and contains a great amount of a soldering ingredientin the reaction residue, and leaves impurities (Pb, Sn salts of organicacid) onto the plated resist to degrade the insulating property of theresist, so that there is a problem in a point that a recovery device forsolid matter is required as a washing machine for production line. Onthe other hand, the self-solder QFP technique has a production problemin a point that the individual mounting components are required to besubjected to a plating.

As mentioned above, the conventional techniques for coping with thesurface mounting of fine pitch accompanied with the rapidly progressingminiaturization of conductor circuit substrates, high-density wiring,miniaturization of surface mounting components and the like areinsufficient in the thickness of the soldered layer and hardly providethe necessary electrical insulating property and are lacking in thereliability of the mounting conductor circuit substrate.

SUMMARY OF THE INVENTION

It is an object of the invention to solve the aforementioned problems ofthe conventional techniques and to provide a technique on asolder-precoated conductor circuit substrate capable of conducting finepitch mounting and having excellent productivity.

The inventors have made various studies in order to realize the aboveobject. As a result, it has been found that a metal layer for theconnection of electronic components having a desired thickness and Sn/Pbratio after the melting can be formed by electroless plating when usingan unhomogeneous reaction of Sn under a high alkali without a reducingagent (See Surface Technology, 16(1982) 265-275, U.S. Pat. No.4,269,625) and a substitution reaction between Cu and Sn or Sn and Pbwith ionization tendency, and the invention has been accomplished.

That is, the invention lies in a solder-precoated conductor circuitsubstrate formed by previously disposing a solder layer required formounting onto conductors for the connection of an electronic component,characterized in that the solder layer is made from an Sn thin filmlayer and a Pb-coated Sn layer in which at least a part of Sn crystalgrains is coated with Pb film.

In a preferable embodiment of the solder-precoated conductor circuitsubstrate, the metal layer formed by the Sn thin film layer and thePb-coated Sn layer, in which at least a part of Sn crystal grains iscoated with Pb film, is melted by heating to form an alloy layer.

Furthermore, the method of producing the solder-precoated conductorcircuit substrate formed by previously disposing a solder layer requiredfor mounting onto conductors for the connection of an electroniccomponent according to the invention is characterized in that the solderlayer is formed through the following steps (a)-(c):

(a) a step of forming an Sn thin film layer onto conductors of aconductor circuit substrate formed for the connection of the electroniccomponent;

(b) a step of selectively precipitating Sn on the Sn thin film layerthrough Sn unhomogeneous reaction to form an Sn crystal layer; and

(c) a step of coating at least a part of Sn crystal grains in the Sncrystal layer with Pb film through Sn-Pb substitution reaction based onionization tendency to form a Pb-coated Sn layer.

Moreover, the solder layer is formed through the following steps(a)-(d):

(a) a step of forming an Sn thin film layer onto conductors of aconductor circuit substrate formed for the connection of the electroniccomponent;

(b) a step of selectively precipitating Sn on the Sn thin film layerthrough Sn unhomogeneous reaction to form an Sn crystal layer;

(c) a step of coating at least a part of Sn crystal grains in the Sncrystal layer with Pb film through Sn-Pb substitution reaction based onionization tendency to form a Pb-coated Sn layer; and

(d) a step of melting the Sn thin film layer and the Pb-coated Sn layerin which at least a part of Sn crystal grains is coated with Pb filmthrough heating and then cooling to form an alloy layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1f are representations of process steps for producing anembodiment of the printed circuit substrate used in the invention, andFIG. 2 is a flow chart illustrating an embodiment of solder supply inthe method according to the invention. FIG. 1g is an enlarged view ofportion g of FIG. 1f. In these figures, numeral 1 is a substrate,numeral 2 an adhesive layer (insulating layer), numeral 3 a resist,numeral 4 a Cu pad, numeral 5 an Sn thin film layer, numeral 6 an Sncrystal layer, and numeral 7 a Pb film.

DETAILED DESCRIPTION OF THE INVENTION

The solder-precoated conductor circuit substrate according to theinvention will be described in detail below.

The invention is not a technique using the conventional electrolesseutectic solder plating solution. In other words, the invention is atechnique that the metal layer forming a desired solder alloy layerthrough melting is previously arranged on conductors for the connectionof the electronic component by separately using Sn electroless platingsolution and Pb electroless plating solution.

Particularly, the invention is characterized in a point of utilizing Snunhomogeneous reaction as an electroless plating technique of Sn crystaland a point of forming the metal layer with an Sn thin film layer and aPb-coated Sn layer in which at least a part of Sn crystal grains iscoated with Pb film.

According to the above construction, the shortage of precipitated filmthickness being a drawback in the substitution-type electroless eutecticsolder plating can be solved, and also the drawback of incompletealloying when using the Sn-Pb film of simple two-layer structure can besolved. As a result, it is possible to reliably conduct not only themounting with a pitch width of 0.3 mm, which is difficult in theconventional technique, but also the narrower pitch mounting.

The metal layer for the connection of the electronic component consistsof the Sn thin film layer and the Pb-coated Sn layer and is desired tohave a composition ratio after melting of 99.9/0.1-80.0/20.0 as Sn/Pbratio. Because, when Sn content exceeds 99.9%, a very high temperatureis required in the melting and hence the damage is caused in theconductor circuit substrate, while when Pb content exceeds 20.0%, thesolder after the melting is apt to be oxidized.

According to the invention, subtractive substrate, additive substrateand the like can be used as the conductor circuit substrate supplyingthe solder. Even when using either of the substrates, high-densitymounting of fine pitch is possible as compared with the conventionaltechnique.

Among them, the additive substrate has a permanent resist inherentthereto and can utilize a self-alignment effect based on the permanentresist, so that the positioning in the assembling of the surfacemounting component becomes easy and also it is advantageously possibleto prevent the formation of solder bridge by the solder dam effect inthe solder reflow after the mounting of the component. Therefore, thesolder-precoated conductor circuit substrate according to the inventionusing the additive substrate is favorable for mounting more fine pitchedelectronic components.

Then, the invention is described with reference to the method ofproducing the solder-precoated conductor circuit substrate.

(1) In the production of the solder-precoated conductor circuitsubstrate according to the invention, a given printed circuit substrateis first obtained by additive method, subtractive method or the like.

As the substrate used in the above circuit substrate, mention may bemade of plastic substrate, ceramic substrate, metal substrate, filmsubstrate and the like. For example, there are included glass-epoxysubstrate, glass-polyimide substrate, alumina substrate, low-temperaturefired ceramic substrate, aluminum nitride substrate, aluminum substrate,iron substrate, polyimide film substrate and so on. These substrates areused to prepare one-side circuit substrate, bothsides through-holecircuit substrate and multi-layer circuit substrate such as Cu/polyimidemulti-layer circuit substrate or the like.

As the method of forming the conductor circuit in the above circuitsubstrate, electroless and electrolytic platings of copper, nickel,gold, silver and the like, sputtering of chromium, molybdenum or thelike, paste printing of copper, silver, palladium, tungusten or the likeare applicable, among which it is preferable to use electroless andelectrolytic platings of copper.

In the production method according to the invention, a layer ofdifferent metal can be supplied onto the conductor circuit formed by theabove various methods.

Moreover, in the method according to the invention, the above conductorcircuit is formed by using various methods practised in known printedcircuit substrates. For example, there are a method of subjecting thesubstrate to an electroless plating and etching a circuit, a method ofdirectly forming a circuit in the electroless plating and the like.

(2) Then, Sn thin film layer is formed on the conductor for theconnection of the electronic component on the conductor circuitsubstrate provided with the above formed conductor circuit by Cu-Snsubstitution reaction desirably based on formation of Cu complex ofthiourea. This step is necessary before a step of Sn unhomogenousreaction plating because the Sn unhomogenous reaction as mentioned laterdoes not occur on the Cu surface but occurs only on Sn.

The thickness of the thin film is desired to be 0.1-2 μm, preferably0.3-0.5 μm. When it is less than 0.1 μm, the Sn unhomogeneous reactiondoes not occur, while when it exceeds 2 μm, it is difficult to form thefilm.

The reason why thiourea is added in this step is due to the fact that,for example, if thiourea is not existent in Cu pad, standard electrodepotential of Cu is higher than standard electrode potential of Sn, sothat substitution precipitation of Sn is not caused on Cu pad. In thispoint, if S (thiourea: SN(NH₂)₂) is existent, Cu forms a thio-complex toshift the standard electrode potential to a value lower than that of Snand hence the substitution precipitation of Sn can be conducted.

Moreover, the metal forming the conductor may include Ni, Au, Ag, Cr, W,Mo and the like in addition to Cu. In this case, the standard electrodepotential of the conductor circuit metal is required to be lower thanthat of Sn.

(3) Then, Sn crystal layer is formed on the Sn thin film layer formed onthe conductor of the circuit substrate for the connection of theelectronic component by electroless plating through unhomogeneousreaction of Sn based on selective precipitation on Sn.

The electroless plating through unhomogeneous reaction of Sn is aself-catalyst type electroless plating caused by unhomogeneous reactionof stannous acid ion from an alkali solution containing no reducingagent:

    2Sn(OH).sub.3.sup.- →Sn+Sn(OH).sub.6.sup.2-

The thus obtained Sn crystal layer is at a state of aggregating crystalgrains owing to coarse precipitated crystals and becomes porous. When atleast a part of Sn crystal grains in the porous crystal layer issubjected to a substitution reaction with Pb, the resulting Pb-coated Snlayer is easy to form a complete alloy layer at a lower temperature ascompared with the simple Sn-Pb two layer structure according to theconventional technique.

The Sn crystal grains are desirable to have an average particle size of1-100 μm, preferably about 50 μm for coping with the formation of finepattern. When the average particle size of the Sn crystal grains is lessthan 1 μm, it is necessary to make the precipitation rate of Sn crystalslow and hence the plating time becomes longer for obtaining Sn layer ,having a desired thickness and the resist for plating is not durable inthe plating bath having a high alkali concentration at high temperature.On the other hand, when the average particle size of the Sn crystalgrains exceeds 100 μm, the surface area of Sn crystal grains to besubstituted with Pb becomes smaller.

The precipitation rate of Sn crystal grains is desirable to be 1-50μm/hr as a conversion by thickness after the melting. When theprecipitation rate is less than 1 μm/hr, the plating resist is notdurable in the high-alkali plating bath at high temperature, while whenthe precipitation rate exceeds 50 μm/hr, the average particle size of Sncrystal grains becomes larger and the surface area to be substitutedwith Pb becomes smaller.

In this case, metal salts of Sn (II) are sufficient as a supply sourceof Sn, which preferably include chlorides (SnCl₂.2H₂ O), acetate (Sn(CH₃COO)₂), borofluoride (Sn(BF₄)₂) and sulfate (SnSO₄).

Moreover, it is difficult to obtain Sn film having a thickness of notless than 20 μm by the conventional electroless plating. In thisconnection, Sn film having a thickness of not less than 20 μm can easilybe obtained by the aforementioned electroless plating using theunhomogeneous reaction.

(4) At least a part of Sn crystal grains in the Sn crystal layer formedby the unhomogeneous reaction of Sn is replaced with Pb by Sn-Pbsubstitution reaction based on ionization tendency to form Pb film,whereby a solder-precoated conductor circuit substrate provided with ametal layer forming the solder layer on the conductor for the connectionof the electronic component is produced.

The Pb-coated Sn layer obtained by substituting a part of Sn crystallayer with Pb through the unhomogeneous reaction is a structure ofcovering the surface of Sn crystal grain with Pb film because the layerstructure of Sn crystal layer is an aggregate of Sn crystal grains.

The Sn-Pb substitution reaction based on the ionization reaction isdesirably conducted at a temperature of from room temperature to 90° C.,preferably about 50° C. When the temperature is lower than roomtemperature, the reaction rate becomes slow, while when it exceeds 90°C., the reaction rate is too fast and it is difficult to control thecomposition of Sn/Pb.

The Pb film formed by replacing at least a part of Sn crystal grainsthrough Sn-Pb substitution reaction based on the ionization tendency isdesirable to have a thickness of 0.1-5 μm, preferably 0.3-3 μm. When thethickness of Pb film is less than 0.1 μm, the solder alloy layer can notbe formed from the Sn thin film layer and the Pb-coated Sn layer, whilewhen the thickness of Pb film exceeds 5 μm, Pb surface is oxidized.

In this case, metal salts of Pb(II) are sufficient as a supply source ofPb, which preferably include chloride (PbCl₂), acetate (Pb(CH₃ COO)₂.2H₂O), borofluoride (Pb(BF₄)₂) and nitrate (Pb(NO₃)₂).

(5) Preferably, the thus produced solder-precoated conductor circuitsubstrate according to the invention is placed in a heating furnace(reflow furnace) and heated to melt the metal layer, whereby the alloyedsolder-precoated conductor circuit substrate is formed.

According to the thus produced solder-precoated conductor circuitsubstrate, solder is melted and resolidified by the heat press of theconnection portion of the electronic component onto the obtained solderlayer and the like, whereby the electronic component can be mounted onthe substrate with a high reliability.

Moreover, the mounting of such an electronic component may be carriedout by previously mounting the electronic compartment onto the alloylayer without alloying the metal layer in the heating furnace (reflowfurnace) and thereafter melting and alloying the metal layer in theheating furnace (fellow furnace).

EXAMPLE 1

(1) Preparation of additive type printed conductor circuit substrate

(a) 1275 parts by weight of melamine resin is mixed with 1366 parts byweight of 37% formalin and 730 parts by weight of water, adjusted topH=9.0 with 10% sodium carbonate, held at 90° for 60 minutes and addedwith 109 parts by weight of methanol to obtain a resin solution.

(b) The resin solution is dried by a spray drying method to obtainpowdery resin.

(c) The resin powder obtained in the item (b), a releasing agent and acuring catalyst are pulverized and mixed in a ball mill to obtain amixed powder.

(d) The mixed powder is placed in a mold heated at 150° C. and heldunder a pressure of 250 kg/cm² for 60 minutes to obtain a moldedproduct. In the molding, the mold is vented.

(e) The molded product obtained in the item (d) is finely pulverized ina ball mill to obtain powders having a particle size of 0.5 μm or 5.5μm.

(f) 60 parts by weight of phenol-novolac type epoxy resin (made by YukaShell K.K.), 40 parts by weight of bisphenol A-type epoxy resin (made byYuka Shell K.K.) and 5 parts by weight of an imidazole curing agent(made by Shikoku Kasei K.K.) are dissolved into butylcellosolve acetateand mixed with 15 parts by weight of the fine powder having a particlesize of 0.5 μm and 30 parts by weight of the fine powder having aparticle size of 5.5 μm in the above item (5) based on 100 parts byweight of solid content in the composition and then kneaded in threerolls and added with butylcellosolve acetate to prepare an adhesivesolution having a solid content concentration of 75%. The viscosity ofthis solution is 5.2 Pa.s at a revolution number of 6 rpm and 2.6 Pa.sat 60 rpm as measured at 20° for 60 seconds by means of a digitalviscosity meter made by Tokyo Keiki K.K. according to JIS-K 7117, and anSVI value thereof (thixotropic property) is 2.0.

(g) A glass-epoxy substrate 1 (see FIG. 1a) is toughened by polishing tohave a roughness of JIS-B0621 Rmax=2-3 μm and then-the adhesive solutionprepared in the item (f) is applied onto the substrate by using a rollcoater (see FIG. 1b). As a coating roll in this application method, acoating roll for middle-high viscosity resist (made by Dainippon ScreenK.K.) is used, in which a gap between the coating roll and a doctor baris 0.4 mm, a gap between the coating roll and a back-up roll is 1.4 mmand a transfer velocity is 400 mm/s. After being left to stand at ahorizontal state for 20 minutes, the drying is carried out at 70° C. toform an adhesive layer 2 having a thickness of about 50 μm (see FIG.1c).

(h) The substrate provided with the adhesive layer 2 (see FIG. 1c) isimmersed in an oxidizing agent consisting of an aqueous solution of 500g/l of chromic acid (CrO₃) at 70° C. for 15 minutes to roughen thesurface of the adhesive layer 2, which is immersed in a neutral solution(made by Shiplay Inc.) and washed with water. A palladium catalyst (madeby Shiplay Inc.) is applied onto the toughened adhesive layer 2 of thesubstrate to activate the surface of the adhesive layer 2 (see FIG. 1d).

(i) The substrate treated in the above item (h) is subjected to a heattreatment for fixation of the catalyst at 120° C. in an atmosphere ofnitrogen gas (10 ppm) for 30 minutes.

(j) Onto the substrate treated in the item (i) is applied a resinsolution obtained by applying a photosensitivity to the adhesivesolution of the item (f) through the roll coater likewise the item (g).In order to remove the solvent in the resulting applied layer, the heattreatment is carried out at 80° C. for 30 minutes, and then exposed to alight through a mask for the formation of a pattern, which is developedwith an Eterna IR (made by Asahi Chemical Industry Co., Ltd.),irradiated by ultraviolet ray (UV cure) and subjected to a heattreatment to form a plating resist 3 (thickness: 40 μm) (see FIG. 1e).

(k) The substrate provided with the plating resist 3 of the item (j) isimmersed in an electroless copper plating solution having a compositionunder conditions as shown in Table 1 for 11 hours to form an electrolesscopper plated film of 25 μm in thickness for the formation of conductorportion, whereby an additive type printed conductor circuit substrateprovided with Cu pad for mounting an electronic component (0.15 mm TAB,0.3.0.5 mm QFP) having various lead pitches (0.15, 0.3, 0.5 mm) (seeFIG. 1f). In this case, a step difference between the resist 3 and theCu pad 4 of the plated film is 15 μm.

                  TABLE 1                                                         ______________________________________                                        Composition and conditions of electroless copper                              plating solution                                                              ______________________________________                                        Copper sulfate        0.06 mol/l                                              Formalin (37%)        0.30 ml/l                                               Sodium hydroxide      0.35 mol/l                                              EDTA                  0.35 mol/l                                              Additive              little                                                  Temperature           70-72° C.                                        pH                    12.4                                                    ______________________________________                                    

(2) Pretreatment

Then, the printed conductor circuit substrate having the electrolesscopper plated Cu pad 4 for the mounting of the electronic component (seeFIG. 2a) is treated with a degreasing solution (Alkykate, made byShiplay Inc.) at 70° C. for 5 minutes, washed with water, treated withan activating solution (usually soft etch) at room temperature for 10seconds to form a solder feeding plated substrate.

(3) Formation of Sn thin film layer 5 (see FIG. 2b)

The substrate obtained in the item (2) is immersed in an electrolessSn-substituted plating solution having a composition and conditionsadjusted by dissolving thiourea and solution of tin borofluoride(Sn(BF₄)₂) in water as shown in Table 1 for about 1 minute to substituteCu surface with Sn layer, whereby an electroless Sn substituted platedfilm of 0.3-0.5 μm in thickness is formed.

                  TABLE 2                                                         ______________________________________                                        Composition and conditions of electroless Sn                                  substitution plating solution                                                 ______________________________________                                        Tin borofluoride      0.1 mol/l                                               Thiourea              1.0 mol/l                                               Temperature           80° C.                                           pH                    1.2                                                     ______________________________________                                    

(4) Formation of Sn crystal layer 6 (see FIG. 2(c))

The substrate subjected to the plating treatment of the item (3) iswashed with water and immersed in an electroless Sn thick platingsolution (utilizing Sn unhomogeneous reaction) having a composition andconditions adjusted by adding a solution of tin chloride (titanouschloride dihydrate) in water to a solution of sodium hydroxide in waterwith stirring and finally adding formalin as a stabilizing agent asshown in Table 3 for 3 hours to form Sn crystal layer on the Sn thinfilm 5 formed in the item (3), whereby the electroless Sn thick platedfilm having a maximum thickness of 100-150 μm is obtained. The obtainedSn thick crystal layer is an aggregate of crystal grains as a result ofobservation on surface and cut section. The size of the crystal grain isabout 50 μm on average. Furthermore, the precipitation rate of Sncrystal grain is 25 μm/hr as a thickness conversion after the melting.

                  TABLE 3                                                         ______________________________________                                        Composition and conditions of electroless Sn                                  thick plating solution                                                        ______________________________________                                        Sodium hydroxide        5.2 mol/l                                             Titanous chloride dihydrate                                                                           0.4 mol/l                                             Formalin (37%)          1.0 ml/l                                              Additive                little                                                Temperature             80° C.                                         pH                      14-15                                                 ______________________________________                                    

(5) Formation of Pb film 7 (see FIG. 2d)

The substrate subjected to the treatment of the item (4) is washed withwater and immersed in an electroless Pb substitution plating solutionhaving a composition and conditions adjusted by dissolving a solution oflead (II) tetrafluoroborate and hydroborofluoric acid into water asshown in FIG. 4 for 5 minutes to form an electroless Pb substitutedplated film skinly replacing the surface of the Sn crystal grain in theSn crystal layer 6 of the item (4) with Pb film (presumption 0.3-3 μm).

                  TABLE 4                                                         ______________________________________                                        Composition and conditions of electroless Pb                                  thick plating solution                                                        ______________________________________                                        Lead (II) tetrafluoroborate                                                                           0.1 mol/l                                             Hydroborofluoric acid   1.0 mol/l                                             Temperature             80° C.                                         pH                      3.5                                                   ______________________________________                                    

(6) Post treatment

Then, the substrate treated in the item (5) is washed with water, driedin a hot air dryer at 80° C. for 10 minutes, an additive type solderprecoated conductor circuit substrate in which metal (Sn, Pb) formounting the electronic component is supplied onto the Cu pad 4 throughplating is obtained (see FIG. 2).

Moreover, the composition of the connecting metal for mounting theelectronic component is Sn/Pb ratio of 6/4 as a result of EDS analysisafter the solder precoated conductor circuit substrate is heated to meltand alloy the metal (Sn, Pb) supplied through single plating.

(7) Mounting of electronic component

In case of 0.3, 0.5 mm QFP, the electronic component is placed on thegiven portion of the additive type solder precoated conductor circuitsubstrate without heating and then heated in a reflowing machine for themounting, while in case of 0.15 mm TAB, the electronic component isplaced on the given portion and heated by a hot bar (pulse heat system)for the mounting.

EXAMPLE 2

(1) The same procedure as shown in the steps of the items (1)-(6) ofExample 1 is carried out to obtain an additive type solder precoatedconductor circuit substrate in which metal for mounting the electroniccomponent (solder after the melting: Sn/Pb ratio is 6/4) is suppliedonto Cu pad through plating.

(2) Then, the circuit substrate is immersed in an organic hot medium at210° C. for 5 seconds to melt and alloy Sn and Pb supplied on copper. Inthis case, the solder alloyed by melting (Sn/Pb ratio: 6/4) is renderedinto an arc shape in which a resist wall portion having Cu lead width orsame height is a cord.

(3) Then, the same procedure as in the step of the item (7) of Example 1is carried out to mount various electronic components.

EXAMPLE 3

(1) The same procedure as shown in the steps of the items (1)-(6) ofExample 1 is carried out except that the substrate is immersed in a Pbsubstitution plating solution for 2 minutes at the step of the item (5)of Example 1, whereby an additive type solder precoated conductorcircuit substrate is obtained.

(2) The circuit substrate is subjected to a heating treatment to conductthe alloying (Sn/Pb ratio: 9/1) and then the same procedure as in thestep of the item (7) of Example 1 is repeated to mount variouselectronic components.

EXAMPLE 4

(1) A copper-lined laminate (thickness of rolled copper foil 18 μm) istreated by a usual etched boil process to obtain a subtractive typeprinted conductor circuit substrate provided with Cu pads for mountingan electronic component of various lead pitches (0.15, 0.3, 0.5 mm).

(2) Then, the same procedure as in the steps of the items (3)-(6) ofExample 1 is repeated to obtain a subtractive type solder precoatedconductor circuit substrate in which metal (solder) for mounting theelectronic component is supplied onto copper and copper side wall.

(3) Then, the circuit substrate is immersed in an organic heat medium at210° C. for 5 seconds to melt and alloy Sn and Pb supplied on copper andcopper side wall. In this case, the solder alloyed by melting (Sn/Pbratio: 6/4) has a shape enveloping the Cu pad.

(4) Then, the same procedure as in the step of the item (7) of Example 1is repeated to mount various electronic components.

Comparative Example 1

(1) The same procedure as in the steps of the item (1) of Example 1 isrepeated to obtain an additive type printed conductor circuit substrateprovided with Cu conductor portions of various pad pitches.

(2) Then, a solder paste having a particle size of 20-38 μm (made byTamura Seisakusho) is printed on the circuit substrate at a squeeze rateof 2-3 cm/s through a metal mask by means of a solder printing machine(made by Yokota Seisakusho) to form a solder of 50 μm (Sn/Pb ratio:6/4). Moreover, the solder cannot be formed on Cu pad having a pitch ofnot more than 0.3 mm with a high accuracy. Further, the scattering ofsolder thickness is caused in accordance with the lead pitch.

(3) Then, the same procedure as in the step of the item (7) of Example 1is repeated to mount various electronic components.

Comparative Example 2

(1) The same procedure as in the steps of the item (1) of Example 1 toobtain an additive type printed conductor circuit substrate providedwith Cu conductor portions of various pad pitches. Moreover, leads forthe supply of current are previously arranged on all pads.

(2) Then, the circuit substrate is subjected to an electrolytic solderplating in the usual manner to form a solder of 50 μm (Sn/Pb ratio: 6/4)on the Cu pad.

(3) Then, the same procedure as in the steps of the item (7) of Example1 is repeated to mount various electronic components.

Comparative Example 3

(1) The same procedure as in the steps of the item (1) of Example 1 toobtain an additive type printed conductor circuit substrate providedwith Cu conductor portions of various pad pitches.

(2) Then, the circuit substrate is pretreated and then subjected to anelectroless solder plating (Cu substitution type) at room temperature.In this case, however, only the solder of 10-15 μm (Sn/Pb ratio: 6/4) isformed on the Cu pad. The thickness of the solder is a limit of theabove value because the formation of the solder through plating utilizesthe substitution reaction with Cu as a pad.

(3) Then, the same procedure as in the steps of the item (7) of Example1 is repeated to mount various electronic components.

Comparative Example 4

(1) The same procedure as in the steps of the item (1) of Example 4 isrepeated to obtain a subtractive type printed conductor circuit boardprovided with Cu conductor portion of various pad pitches.

(2) Then, the circuit substrate is subjected to the same treatment as inthe step of the item (2) of Comparative Example 1 , whereby the solderpaste is printed on the substrate to form a solder on the Cu pads ofvarious lead pitches. Similarly, the solder can not be formed on the Cupad having a pitch of not more than 0.3 mm with a high accuracy and thescattering of solder thickness is caused.

(3) Then, the same procedure as in the steps of the item (7) of Example1 is repeated to mount various electronic components.

Comparative Example 5

(1) The same procedure as in the steps of the item (1) of Example 4 isrepeated to obtain a subtractive type printed conductor circuit boardprovided with Cu conductor portion of various pad pitches. Moreover,leads for the supply of current are previously arranged on all pads.

(2) Then, the circuit substrate is subjected to the same treatment as inthe step of the item (2) of Comparative Example 2 to form a solder of 50μm (Sn/Pb ratio: 6/4) on the Cu pad of the substrate through theelectrolytic solder plating.

(3) Then, the same procedure as in the steps of the item (7) of Example1 is repeated to mount various electronic components.

Comparative Example 6

(1) The same procedure as in the steps of the item (1) of Example 4 isrepeated to obtain a subtractive type printed conductor circuit boardprovided with Cu conductor portion of various pad pitches.

(2) Then, the circuit substrate is subjected to the same treatment as inthe step of the item (2) of Comparative Example 3 to form a solder of10-15 μm (Sn/Pb ratio: 6/4) on the Cu pad of the substrate through theelectroless solder plating.

(3) Then, the same procedure as in the steps of the item (7) of Example1 is repeated to mount various electronic components.

After the solder is supplied onto the Cu pad by using various soldersupplying methods (method according to the invention, solder pasteprinting method, electrolytic solder plating method and electrolesssubstitution solder plating method) alone to form the solder precoatedconductor circuit substrate as mentioned above and then electroniccomponents having various lead pitches (0.15TAB, 0.3QFP, 0.5QFP) aremixedly mounted thereonto, the mounting reliability is measured toobtain results shown in Table 5. As seen from the results of this table,it has been confirmed that it is possible to conduct surface mountingwithin a wide range (rough pitch, fine pitch) in the mounting using thesolder precoated conductor circuit substrate according to the invention.

Moreover, in case of the electrolytic solder plating in the comparativeexample in Table 5, it is required to use leads for the supply ofcurrent, so that there are problems that the decrease of wiring densityis brought about and the productivity is very poor. Furthermore, in caseof the electroless substitution solder plating, it is difficult tothicken the solder layer and it is necessary to supply the solder ontothe Cu pad of rough pitch by any means and there is a problem that theproductivity becomes very poor. Moreover, in case of the solder pasteprinted method, there is a problem that it is difficult to supply thesolder onto the Cu pad of fine pitch (not more than 0.3 mm) with a highreliability.

In the method according to the invention, it is clear that there are notcaused the aforementioned problems and the invention is a very usefultechnique.

                                      TABLE 5                                     __________________________________________________________________________           Kind         Mounting                                                         of           reliability  Thickness                                                                           Correspondence                                circuit                                                                            Solder  *2           of    to fine Electrolytic                          substrate                                                                          supply  0.5 mm                                                                            0.3 mm                                                                            0.15 mm                                                                            solder                                                                              pitch   lead                                  *1   method  QFP QFP QFP  *3    *4      *5                             __________________________________________________________________________    Example                                                                       1      A    Method  ∘                                                                     ∘                                                                     ∘                                                                      ∘                                                                       ∘                                                                         ∘                  2      A    according                                                                             ∘                                                                     ∘                                                                     ∘                                                                      ∘                                                                       ∘                                                                         ∘                  3      A    to the  ∘                                                                     ∘                                                                     ∘                                                                      ∘                                                                       ∘                                                                         ∘                  4      S    invention                                                                             ∘                                                                     ∘                                                                     ∘                                                                      ∘                                                                       ∘                                                                         ∘                  Comparative                                                                   Example                                                                       1      A    Solder paste                                                                          ∘                                                                     x   x    ∘                                                                       x       ∘                              printing methd                                                    2      A    Electrolytic                                                                          ∘                                                                     ∘                                                                     ∘                                                                      ∘                                                                       ∘                                                                         x                                          solder plating                                                                method                                                            3      A    Electroless                                                                           x   x   ∘                                                                      x     ∘                                                                         ∘                              substitution                                                                  solder plating                                                                method                                                            4      S    Solder paste                                                                          ∘                                                                     Δ                                                                           x    ∘                                                                       x       ∘                              printing method                                                   5      S    Electrolytic                                                                          ∘                                                                     ∘                                                                     ∘                                                                      ∘                                                                       ∘                                                                         x                                          solder plating                                                                method                                                            6      S    Electroless                                                                           x   x   ∘                                                                      x     ∘                                                                         ∘                              substitution                                                                  solder plating                                                                method                                                            __________________________________________________________________________     *1: A  Additive type printed conductor circuit substrate                      S  Subtractive type printed conductor circuit substrate                       *2: Mounting reliability                                                      Mounting reliability when the solder supply method is restricted to one       method and 0.5 mm QFP, 0.3 mm QFP and 0.15 mm TAB are mixedly mounted ont     the same substrate, which is evaluated by a product yield after the           mounting.                                                                     ∘; 100%  Δ; 98-100%  X; less than 98%                       *3: Thickness of solder                                                       It is evaluated whether or not the formation of solder having a thickness     of not less than 50 μm is possible. ∘; possible  X;            impossible                                                                    *4: Correspondence to fine pitch                                              It is evaluated whether or not the formation of solder having a pitch of      not more than 0.3 mm is possible. ∘; possible  X; impossible      *5: Electrolytic lead                                                         It is evaluated whether or not leads for the supply of current is             possible. ∘; necessary  X: unnecessary                       

INDUSTRIAL APPLICABILITY

As mentioned above, the solder precoated conductor circuit substratesaccording to the invention have excellent properties as compared withthe conventional surface mounting substrates and are possible to conductthe mounting at more fine pitch and are excellent in the productivity,so that they are used in not only the mounting at a pitch width of 0.3mm but also in the mounting at narrower pitch in a high reliability.

Particularly, the solder precoated conductor circuit substrate using theadditive substrate has a peculiar permanent resist, so that aself-alignment effect inherent to the permanent resist can be utilized.As a result, it is easy to conduct the positioning in the assembling ofsurface mounting component and it is possible to prevent solder bridgethrough a solder dam effect in the solder fellow after the mounting ofthe component.

We claim:
 1. A solder-precoated conductor circuit substrate formed bypreviously disposing a solder layer required for mounting ontoconductors for the connection of an electronic component, said solderlayer comprising an Sn thin-film layer and a Pb-coated Sn layer in whichat least a part of Sn crystal grains is coated with Pb film.
 2. Thesolder-precoated conductor circuit substrate according to claim 1,wherein a metal layer formed by the Sn thin-film layer and the Pb-coatedSn layer, in which at least a part of Sn crystal grains is coated withPb film, is melted by heating to form an alloy layer.
 3. Thesolder-precoated conductor circuit substrate according to claim 1,wherein the Sn thin-film layer has a thickness of 0.1-2 μm.
 4. Thesolder-precoated conductor circuit substrate according to claim 1,wherein the Sn crystal grains in the Pb-coated Sn layer has an averageparticle size of 1-100 μm.
 5. The solder-precoated conductor circuitsubstrate according to claim 1, wherein the Pb film in the Pb-coated Snlayer has a thickness of 0.1-5 μm.
 6. A method of producing asolder-precoated conductor circuit substrate formed by previouslydisposing a solder layer required for mounting onto conductors for theconnection of an electronic component, comprising forming the solderlayer by a process comprising:(1) forming an Sn thin-film layer ontoconductors of a conductor circuit substrate formed for the connection ofthe electronic component; (2) selectively precipitating Sn on the Snthin-film layer through Sn unhomogeneous reaction to form an Sn crystallayer; and (3) coating at least a part of Sn crystal grains in the Sncrystal layer with Pb film through Sn-Pb substitution reaction based onionization tendency to form a Pb-coated Sn layer.
 7. A method ofproducing a solder-precoated conductor circuit substrate formed bypreviously disposing a solder layer required for mounting ontoconductors for the connection of an electronic component, comprisingforming the solder layer by a process comprising:(1) forming an Snthin-film layer onto conductors of a conductor circuit substrate formedfor the connection of the electronic component; (2) selectivelyprecipitating Sn on the Sn thin-film layer through Sn unhomogeneousreaction to form an Sn crystal layer; and (3) coating at least part ofSn crystal grains in the Sn crystal layer with Pb film through Sn-Pbsubstitution reaction based on ionization tendency to form a Pb-coatedSn layer; and (4) melting the Sn thin-film layer and the Pb-coated Snlayer in which at least a part of Sn crystal grains is coated with Pbfilm through heating and then cooling to form an alloy layer.
 8. Themethod of producing a solder-precoated conductor circuit substrateaccording to claim 6, wherein the formation of Sn thin-film layer on theconductor for the connection of the electronic component comprises aCu-Sn substitution reaction based on Cu complex formation from Cu andthiourea.
 9. The method according to claim 8, wherein the Sn thin-filmlayer has a thickness of 0.1-2 μm.
 10. The method according to claim 6,wherein the Sn crystal grains in the Pb-coated Sn layer has an averageparticle size of 1-100 μm.
 11. The method according to claim 6, whereina precipitation rate of the Sn crystal layer is 1-50 μm/hr as aconversion into thickness after the melting.
 12. The method according toclaim 6, wherein the Sn-Pb substitution reaction based on ionizationtendency is carried out at room temperature--90° C.
 13. The methodaccording to claim 6, wherein the Pb film in the Pb-coated Sn layer hasa thickness of 0.1-5 μm.